Sterilization with a germicidal agent, such as ethylene oxide gas or ethylene oxide gas mixtures, has played an increasingly important role in sterilizing heat or moisture sensitive materials. Rapid growth in the use of sterile, disposable medical devices is just one consequence of gaseous sterilization with agents such as ethylene oxide. The basic gaseous sterilization process consists of evacuating the sterilization chamber, preconditioning the articles to be sterilized at an optimal relative humidity, generally between 20-70% RH, admitting the sterilizing gas at an appropriate pressure and temperature, maintaining contact between the sterilizing atmosphere and the articles to be sterilized for an appropriate time and finally discharging and evacuating the chamber to remove the sterilant gas.
Although there are many variations on the basic process, the major factors which have to be controlled in order to effect the sterilization are exposure time, temperature, ethylene oxide pressure or partial pressure and relative humidity. The following prior art references provide a good description of the standard sterilization processes and apparatus with which the gaseous sterilizing agents of the invention are useful: "Principles and Method of Sterilization," pp. 501-530, 2nd Ed. (1969) by J. J. Perkins; "Ethylene Oxide Gaseous Sterilization for Industrial Applications," pp. 181-208, in Industrial Sterilization International Symposium, 1972; U.S. Pat. No. 3,068,064 and U.S. Pat. No. 3,589,861.
Ethylene oxide by itself is an extremely flammable gas, its flammability range extends from about 3.5% by volume to 100% by volume in air. When using ethylene oxide alone as a sterilizing gas, precautions such as explosion proof equipment are mandatory.
A preferable practice is to blend the ethylene oxide with another fluid which is inert as far as the sterilizing process is concerned, but serves to dilute the ethylene oxide and render the mixture as a whole nonflammable. Two such blends which have been used as sterilizing gases are dichlorodifluoromethane (CFC-12)/ethylene oxide and carbon dioxide/ethylene oxide. These blends are non-azeotropic in nature and therefore suffer the disadvantage of segregation during vaporization which could lead to potentially flammable or explosive situations if process flow rates, outage volumes, etc. are not closely monitored and controlled.
The CFC-12/ethylene oxide blend is generally supplied as a liquid mixture consisting of 88% by weight CFC-12 and 12% by weight ethylene oxide. This composition is below the critical flammability composition of about 14-15% by weight ethylene oxide in CFC-12, and is therefore nonflammable. A typical hospital sterilization process which utilizes the CFC-12/ethylene oxide blend is performed by evacuating the chamber to about 20-24 inches of mercury vacuum and filling the chamber to about 10 psig pressure with the gas mixture after completinq the humidification step. Sterilization is generally performed around 130.degree. F. This procedure provides up to about 630 milligrams of ethylene oxide per liter.
A disadvantage of using CFC-12 in such mixtures is that fully halogenated chlorofluorocarbons such as CFC-12 are suspected of causing environmental problems in connection with the earth's protective ozone layer.
Although the major purpose of the inert component in these sterilizing gas mixtures is to mask the flammability characteristics of ethylene oxide, simple substitution of an arbitrary nonflammable diluent does not necessarily ensure a useful sterilizing gas mixture. First, the flammability properties of the blend must be such that sufficient ethylene oxide (mg/liter at a typical pressure and temperature) is delivered by the blend to affect the sterilization in an appropriate time. If the diluent does not mask the flammability to a sufficient extent, a lower concentration of ethylene oxide must be used to ensure nonflammability, and either a longer time period is required to perform the sterilization, which affects productivity, or greater operating pressures are required to increase the effective ethylene oxide density in the sterilization chamber. Increasing the operating pressure is generally not a viable option because existing sterilization chambers may not be rated for the increased pressure and, as pointed out by Gunther in U.S. Pat. No. 3,589,861, increased pressure can lead to swelling and rupture of the sealed plastic bags commonly used to package disposable medical devices. Indeed, lower operating pressures are advantageous in this respect.
A candidate inert diluent should preferably also be miscible with ethylene oxide in the liquid phase and should not be too highly volatile that it would segregate from the ethylene oxide to any great extent during vaporization. Segregation or fractionation can lead to potentially flammable or explosive situations. An azeotrope-like mixture would be useful in this context as it does not fractionate by normal evaporation or distillation processes thereby resulting in release of the flammable ethylene oxide component.
It is accordingly an object of this invention to provide a novel sterilizing gas mixture containing ethylene oxide.
It is another object of the invention to provide such a sterilizing gas mixture which contains an inert fluorocarbon diluent which is considered to be stratospherically safe.
Another object of the invention is to provide such a sterilizing gas mixture in which the fluorocarbon is miscible with the ethylene oxide and which mixture is azeotropic or non-segregating.
Still another object of the invention is to provide a novel sterilizing gas mixture which incorporates all of the above stated objectives.
Other objects and advantages of the invention will become apparent from the following description of the invention.